The invention relates to a power steering apparatus, and in particular, to such apparatus in which a control valve which controls the delivery to or displacement from a power cylinder of hydraulic fluid is of a disc type.
Generally, a control valve of disc type comprises a first valve element which is integrally mounted on the input shaft which is associated with a steering wheel, and a second valve element which is integrally mounted on the output shaft which is associated with steerable road wheels and disposed across and in surrounding relationship with the first valve element. In operation, on angular displacement occurs between the input and the output shaft or between the first and the second valve element from their neutral relative position, controlling the delivery to or displacement from a power cylinder of hydraulic fluid. A reaction spring in the form of a torsion bar, a leaf spring, a coiled spring or the like is interposed between the both valve elements in order to enable them to be maintained in their neutral position whenever the valve is not operated.
However, the use of a torsion bar results in an increase in the axial length of the power steering apparatus and in the diameter of a portion thereof which contains the control valve, and also suffers from the inability that it cannot be preloaded. A construction which incorporates a coiled spring in order to enable a preloading results in a complex arrangement. The use of a leaf spring or coiled spring suffers from a disadvantage that a high spring rate cannot be obtained for its size. A proposal has been made to add to the resilience of the spring by producing a reaction oil pressure within the control valve. However, the use of the reaction oil pressure within the control valve of disc type results in a complex construction and also requires additional seals, giving rise to the likelihood that the performance of the control valve may be degraded.